1. Field of the Invention
This invention relates to a drum brake device with an automatic shoe clearance adjustment device for a motor vehicle and, more specifically, to a means to prevent over-adjustment operation of the automatic shoe clearance adjustment device.
2. Description of Related Art
Conventional means to prevent over-adjustment of shoe-to-drum clearances with the automatic shoe clearance adjustment device may broadly be known in the following two types of different means.
For example, as a first type, the Japanese Patent Application Publication Numbers 57-8975 and 2-15737 disclose one means disabling automatic adjustment operation of shoe-to-drum clearances by inhibiting a rotation of an adjustment lever, when an actuating force to separate brake shoes apart, reaches a predetermined magnitude.
As a second type, the U.S. Pat. Nos. 4,385,681, 4,390,086, and German Patent Application Unexamined Publication Number 3100678 disclose another means disabling automatic adjustment operation of the shoe-to-drum clearances by thermally deforming a bimetallic piece when a brake temperature rises up to the certain degree.
However, both conventional means have various problems as stated below when the respective means is used independently.
Recently, in terms of an effective use of a rear braking force, automobile industries tend to increase the number of motor vehicles with the Electronic Brakeforce Distribution (EBD) system which is intended to provide the most effective braking force distribution in relation to the front and rear braking forces. A vehicle with the EBD system is characterized in that the rear braking force is adapted to be stronger (higher) than that of a conventional vehicle equipped with a hydraulic pressure control valve, i.e., a pressure proportioning valve, a deceleration responsive valve, and a load sensitive proportioning valve.
Due to the fact that the higher load is placed on the rear brake of the vehicle with the EBD system than that of the vehicle without the EBD system, higher thermal energy is generated during the braking operation.
To achieve the required shoe-to-drum clearances under high load and temperature, the shoe-to-drum clearances must be a bit larger than an ideal value during the normal use if one of the above-two means to prevent over-adjustment is employed independently. As a result, if the shoe-to-drum clearances become smaller than the setting, it is less likely to cause a problem of dragging. Although it is possible to prevent the over-adjustment of the shoe-to-drum clearances by designing the shoe-to-drum clearances to be larger during the normal use, it automatically leads to a longer pedal stroke giving an uncomfortable feeling to the driver.
As a general strategy to deal with the above-problems in relation to the high load and high temperature condition, it is known that a diameter of the brake drum may be designed to be larger, which also enlarges the size of drum brake itself. While an enlarged brake may resolve the above-problems, the enlarged brake generally causes other problems of providing a heavier device, an environmentally problematic device, and a high energy consuming device. These problems lead to the need for making an improvement without enlarging the size of the brake.